1. Field of the Invention
The present invention relates to an exhaust gas purifying system such as a urea SCR (selective catalyst reduction) system as a typical example, capable of purifying exhaust gas emitted from an internal combustion engine, for example, mounted on motor vehicles, by carrying out exhaust purifying reaction on catalysts using a predetermined additive agent. More particularly, the present invention relates to an exhaust gas purifying system capable of purifying exhaust gas by exhaust gas purifying reaction as well as promoting the exhaust gas purifying reaction with an additive agent such as a urea solution to be injected into the exhaust gas flow by an additive agent injection valve.
2. Description of the Related Art
Recently, urea SCR (selective catalyst reduction) system as an exhaust gas purifying system has been studied and developed, which is applicable to thermal power plants, various types of factories, and auto vehicles (in particular, a motor vehicle equipped with a diesel engine). The urea SCR system is capable of purifying particulate matter (PM) such as NOx (Nitrogen oxide) contained in the exhaust gas. Some of those researches have been put into practical use.
There is a conventional well-known urea SCR system disclosed in Japanese paten laid open publication No. JP 2003-293739.
A description will now be given of a typical configuration, with reference to FIG. 14, adopted to conventional usual urea SCR systems as well as to the system of Japanese patent laid open publication No. JP 2003-293739.
As shown in FIG. 14, the conventional urea SCR system is composed mainly of a catalyst unit 51, an exhaust tube 52, and an additive agent injection valve 53 placed in the middle of the exhaust tube 52. The catalyst unit 51 promotes the exhaust gas purifying reaction. Through the exhaust gas tube 52 the exhaust gas emitted from an exhaust gas generation source (for example, an engine) flows and is introduced into the catalyst unit 51. The additive agent injection valve 53 injects urea solution (as additive agent) to the exhaust gas flowing in the exhaust tube 52. The catalyst 51 is capable of promoting reduction reaction for NOx contained in the exhaust gas.
In the conventional urea SCR system having the above configuration shown in FIG. 14, the additive agent injection valve 53 injects the urea solution into the flow of the exhaust gas in the exhaust tube 52, the flow of the exhaust gas (or the exhaust gas flow) supplies the injected urea solution to the catalyst unit 51 which houses catalyst at the downstream side when observed from the position of the additive injection valve 53.
In the catalyst unit 51, the reduction reaction of NOx contained in the exhaust gas is performed with catalyst. This reduction reaction can purify NOx contained in the exhaust gas. In the reduction reaction, urea solution is hydrolyzed with thermal energy of the exhaust gas to generate ammonia (NH3). NOx contained in the exhaust gas is reduced and purified with the catalyst in the catalyst unit 51. Japanese patent kohyo (Japan unexamined patent publication as a national publication of translated version) No. JP 2001-516635 disclosed such an additive agent injection valve 53 shown in FIG, 14 which is mounted on the exhaust gas tube 52, more particularly, disclosed the additive agent injection valve mounted on a passage wall of the exhaust gas tube 52. Like the configuration shown in FIG. 14, the Japanese patent kohyo No. JP 2001-516635 disclosed the urea addition valve placed in an exhaust gas tube so that an outlet opening of the urea addition valve leans to an exhaust gas tube toward a catalyst unit which houses catalyst. However, it is generally desirable to spray ammonium gas generated by hydrolysis of urea solution onto the entire end surface of the catalyst unit 51 at the upstream side of the exhaust gas flow in order to adequately purify the exhaust gas in the catalyst unit 51. This configuration enables that NOx contained in the exhaust gas is completely reduced in the entire area of the catalyst unit 51. Accordingly, there is a demand to spray the additive agent (urea solution) into the entire section area of the exhaust gas tube by the additive agent injection valve in order to distribute ammonium gas into the entire end surface side of the catalyst unit 51 at the upstream side thereof. In particular, in a case of mounting the additive agent injection valve 53 on the exhaust gas tube 52 as an exhaust gas passage of a straight pile shape, as shown in FIG. 15, most of spray immediately following the injection by the additive agent injection valve 53 is carried away with the flow of the exhaust gas emitted from an internal combustion engine, as a result, there is a possibility that the spray does not reach the opposite part 60 in position to the area at which the additive agent injection valve 53 is mounted on the exhaust gas tube 52. This causes that ammonium gas generated by hydrolysis of urea solution reaches only a part of the upstream side end surface of the catalyst unit 51. This means that the catalyst does not serve as catalyst in a part of the catalyst unit at the upstream side thereof to which no ammonium gas is carried and where NOx is not reduced and purified. Thus, there is a possibility to adequately exhibit the purifying performance of the catalyst at the part to which no ammonium gas is carried.
By the way, Japanese patent laid open publication No. JP 2001-516635 disclosed a net (designated by reference number 56 shown in FIG. 3 of JP 2001-516635) placed at an outlet opening of an additive agent injection valve in order to uniformly spray urea solution into an exhaust gas tube, and further disclosed a deflector (designated by reference number 28 shown in FIG. 2 of JP 2001-516635) in order to change the direction of the urea solution injected from the additive agent injection valve.
However, spray of urea solution injected from the additive agent injection valve, as shown in the conventional system disclosed in JP 2001-516635, is also considerably influenced by the exhaust gas flow. This case also introduces a possibility of not reaching the spray of the urea solution to the opposite area to the additive agent injection valve mounted on the exhaust tube.